1. Field of the Invention
The present invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program for implementing broadband wireless transmission between a plurality of wireless stations, as typically performed in a wireless LAN (Local Area Network) or PAN (Personal Area Network). In particular, the invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program that expand transmission capacity by carrying out MIMO (Multi Input Multi Output) communication forming multiple logical channels between a pair of a transmitter with multiple antennas and a receiver with multiple antennas by using spatial multiplexing.
More specifically, the invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program that carry out MIMO transmission using an enormous number of transmit/receipt antenna pairs to obtain significantly large transmission capacity. In particular, the invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program that perform spatial multiplexing using a more accurate receive weight obtained by avoiding the effect of transmission channel variations associated with the expansion of an area for channel matrix acquisition, in a large-number MIMO communication system having an enormous number of transmit/receipt antenna pairs.
2. Description of the Related Art
In recent years, there has been conducted active research and development on the wireless LAN or PAN typified by IEEE 802.11 and IEEE 802.15. The IEEE 802.11a standard supports a modulation scheme achieving a communication speed of up to 54 Mbps. However, there is a need for additional standards that can realize a higher bit rate. IEEE 802.11n aims to develop a wireless LAN technology that allows for an effective throughput higher than 100 Mbps and to establish next-generation wireless LAN standards.
As a technology for realizing higher-speed wireless communication, MIMO (Multi-Input Multi-Output) communication is coming to attention. In the MIMO communication scheme, a transmitter distributes transmit data to multiple antennas and transmits it through multiple virtual MIMO channels, and a receiver obtains receive data by processing signals received by multiple antennas. In this manner, the MIMO communication scheme utilizes channel characteristics and differs from a simple transmission/reception adaptive array. The MIMO communication can increase the channel capacity according to the number of antennas without increase of the frequency band and accordingly has higher efficiency of frequency utilization.
FIG. 5 conceptually shows the configuration of a MIMO communication system. As shown in FIG. 5, each of a transmitter and a receiver is equipped with multiple antennas. The transmitter space-time encodes multiple transmit data streams, multiplexes the encoded data, distributes the multiplexed signals to M antennas, and transmits them onto multiple MIMO channels. The receiver receives the multiplexed transmit signals by N antennas through the MIMO channels and space-time decodes the received transmit signals to obtain receive data. In this case, the channel model is composed of a radio environment around the transmitter (transfer function), a channel space structure (transfer function), and a radio environment around the receiver (transfer function). The number of MIMO channels obtained in the MIMO communication system generally matches the number of transmit antennas M or the number of receive antennas N, whichever is smaller, min[M, n].
Before transmitting the multiplexed signals, the transmitter transmits a training signal, e.g., for each antenna in a time-division manner, with which the receiver performs channel estimation. At the other end, the receiver performs channel estimation at a channel estimation unit using the training signal and calculates a channel information matrix H corresponding to the antenna pairs. Based on the inverse matrix H−1 of the obtained channel information matrix H, the receiver obtains a receive weight.
After the training of the receive weight, the transmitter space-time encodes multiple transmit data streams, multiplexes the encoded data, distributes the multiplexed signals to M antennas, and transmits them onto multiple MIMO channels. The receiver receives the multiplexed transmit signals by N antennas through the MIMO channels and space-time decodes the received transmit signals to spatially demultiplex the receive data of each MIMO channel. Multiplexed signals transmitted from the antennas involve crosstalk. However, the receiver can correctly extract the spatially multiplexed signals without crosstalk through appropriate signal processing using the receive weight obtained from the inverse matrix H−1 of the channel information matrix H, that is, improve the signal-to-noise ratio to enhance the degree of certainty of decoding.
While various schemes of MIMO transmission configuration have been proposed, it is a significant problem in implementation how to exchange channel information between the transmitter and the receiver in accordance with an antenna configuration.
In the case of exchanging channel information, it is easy to implement a method of transmitting known information (preamble information) only from the transmitter to the receiver. In this case, the transmitter and the receiver perform spatial multiplexing transmission independently of each other. This is called an open-loop MIMO transmission scheme. As an extension of the open-loop type, there is a closed-loop MIMO transmission scheme for creating ideal, spatially orthogonal channels between the transmitter and the receiver by feedback of preamble information from the receiver to the transmitter as well.
The open-loop MIMO transmission scheme can include a V-BLAST (Vertical Bell Laboratories Layered Space Time) scheme (e.g., see patent document 1). The transmitter simply multiplexes a signal for each antenna and transmits, without providing an antenna weighting factor matrix. In other words, a feedback procedure for obtaining the antenna weighting factor matrix is all omitted. Before transmitting the multiplexed signals, the transmitter inserts a training signal, e.g., for each antenna in a time-division manner, with which the receiver performs channel estimation. At the other end, the receiver performs channel estimation at a channel estimation unit using the training signal and calculates a channel information matrix H corresponding to the antenna pairs. By combing zero-forcing and canceling neatly, a signal-to-noise ratio is improved by utilizing a degree of freedom of each antenna that is caused by the canceling and the degree of certainty of decoding is enhanced.
As an ideal form of the closed-loop MIMO transmission, there is known an SVD-MIMO scheme using the singular value decomposition (SVD) of a propagation function (e.g., see non-patent document 1). In the SVD-MIMO transmission, UDVH is obtained by performing the singular value decomposition of a numerical matrix whose elements denote channel information corresponding to respective antenna pairs, namely a channel information matrix H, and a transmit antenna weighting factor matrix V and a receive antenna weighting factor matrix UH are applied. Thereby, each MIMO channel is expressed as a diagonal matrix D having the diagonal elements represented by the square root of a singular value λi of the ith spatial channel and a signal can be multiplexed to be transmitted without any crosstalk. In this case, it is possible to realize spatially divided (i.e., spatially orthogonal multiplexed), logically independent, multiple transmission channels at both the transmitter and the receiver. According to the SVD-MIMO transmission scheme, it is possible to achieve maximum channel capacity in theory. For example, if the transmitter and the receiver have two antennas each, it is possible to acquire double the transmission capacity at maximum.
In the case of constructing a wireless network in a room, there is formed a multipath environment in which the receiver receives the superposition of direct waves and multiple reflected waves and delayed waves. Principal countermeasures against the delay distortion can include a multicarrier transmission scheme, typified by OFDM (Orthogonal Frequency Division Multiplexing). For example, IEEE 802.11a/n which is a MIMO-transmission-applied LAN system adopts the OFDM modulation scheme.
As described above, in the MIMO communication system irrespective of the open-loop type or the closed-loop type, the basic operation of the receiver is to acquire a channel matrix H using reference signals transmitted from the transmitter and performs spatial demultiplexing using the inverse matrix H−1 of the acquired channel matrix H as the receive weight. In the closed-loop type such as SVD, the transmit antenna weighting factor matrix V is used as a transmit weight at the transmitter. On the other hand, the open-loop type is constructed basically in the same manner as the closed-loop type with the exception of substituting an identity matrix for the transmit antenna weighting factor matrix V.
Consideration will be given to the operation in which the receiver performs training on a receive weight using reference signals transmitted from the transmitter.
FIG. 2 schematically shows an example of the structure of packets transmitted from a MIMO transmitter. In FIG. 2, the MIMO communication system is assumed to have four transmit antennas and four receive antennas, i.e., a 4×4 antenna configuration, and have four reception branches. The reception branches are independent channels corresponding to space streams, namely, MIMO channels.
The transmitter transmits the same synchronization signal through each antenna. Next, the transmitter transmits reference signals with which the receiver performs channel estimation, in a time-division manner from respective transmit antennas. At this time, the reference signals are transmitted in order of a reference signal 1, a reference signal 2, a reference signal 3, and a reference signal 4. Then, the transmitter transmits spatially multiplexed user data of each MIMO channel.
The MIMO receiver uses a receive weight in order to spatially demultiplex receive signals. The receiver performs channel estimation using the reference signals from the antennas, acquires the channel matrix H whose column vectors are channel transfer functions obtained from the reference signals, and obtains the inverse matrix H−1 of the channel matrix H. In this manner, the receive weight can be obtained.
However, the state of a transmission channel changes every moment because of a change in a reflected path due to a move of a person or a device. Therefore, in the case of transmitting respective reference signals in a time-division manner as described above, there is a problem that channel estimation is performed on a different transmission channel due to the time difference between transmission and reception.
In the case of a 4×4 MIMO communication system, four reference signals are transmitted in a time-division manner as shown in FIG. 2. For example, in an OFDM-MIMO communication system operating in the 5 GHz band, one reference signal uses two OFDM symbols at most. Since there are four reference signals transmitted in the MIMO communication system having the configuration of 4×4, eight OFDM symbols are used to obtain a channel matrix H. The eight OFDM symbols correspond to approximately 32 μs. Although the channel matrix H changes every moment, it can be considered that there is almost no change within a short time such as 32 μs.
On the other hand, the MIMO communication system can form MIMO channels that correspond to the number of transmit antennas M or the number of receive antennas N, whichever is smaller, min [M, n] (as described above). Accordingly, the MIMO communication system uses spatial multiplexing acquiring more MIMO channels by increasing the number of transmit/receive antenna pairs, thereby making it possible to greatly expand transmission capacity in theory. Hereinafter, such a MIMO communication system is referred to as a “large-number MIMO”.
In the large-number MIMO communication system, in the case where the transmitter transmits reference signals in a time-division manner from respective transmit antennas and the receiver performs training of the channel matrix, the transmission/reception time difference between the first reference signal and the last reference signal cannot be neglected and it is difficult to acquire an effective channel matrix due to the effect of transmission channel variations. For example, in the case of a MIMO communication system having the configuration of 100×100, an area for channel matrix acquisition requires as much as 800 μs. For this reason, the receive weight becomes inaccurate and it becomes difficult to perform spatial demultiplexing, so that the MIMO communication system cannot offer its performance.
[Patent document 1] Japanese Patent Application Laid-Open No. 10-84324
[Non-patent document 1] http://radio3.ee.uec.ac.jp/MIMO(IEICE_TS).pdf (as of Oct. 24, 2003)